1. Field of the Invention
The present invention is a process for the separation of chlorosilanes from gas streams, in which chlorosilane containing gas streams are brought into contact with water vapor in a first stage and with liquid water in a second stage.
2. Discussion of the Background
Chlorosilanes and hydrogen are formed in the reaction of silicon with hydrogen chloride. This reaction is termed the direct synthesis of chlorosilanes. By cooling the reaction gases of this reaction, a liquid chlorosilane mixture may be condensed. However, even at condensation temperatures far below 0xc2x0 C., chlorosilanes which are liquid at room temperature, i.e., trichlorosilane (silicochloroform) and tetrachlorosilane (silicon tetrachloride), still have a considerable vapor pressure, and therefore significant amounts of these chlorosilanes may remain in the hydrogen-rich waste gas. In addition, small amounts of chlorosilanes which are gaseous at room temperature, i.e., dichlorosilane and optionally monochlorosilane, are also formed in the reaction, and may be present in the waste gas. All of these chlorosilanes must be separated from the hydrogen-rich waste gas before the waste gas may be burned, with or without energy recovery, or used in some other way.
EP 0 089 783 A2 describes a process for the treatment of liquid, chlorosilane-containing waste products or byproducts from the preparation of silicones by hydrolysis of organochlorosilanes in an aqueous medium. In this process, the average Sixe2x80x94Cl functionality of the waste products or byproducts is at least 2.8, and it is intended that the hydrolyzate be recovered in the form of a granulated gel. However, it has been shown that such gas-scrubbing processes simply transform a waste gas problem into a waste water problem, because gelatinous oligomers which are extremely difficult to filter remain in the washing water.
EP 0 532 857 A1 describes a process for steam hydrolysis of the residues produced by a chlorosilane distillation process, which in its continuous embodiment is carried out at an initial temperature of at most 160xc2x0 C. and at a final temperature of at least 170xc2x0 C.
There is therefore a need for a continuous process in which chlorosilanes can be removed simply and reliably from gas streams by hydrolysis, and the hydrolysis product may be produced in a form which can be easily disposed of without problems. Preferably, such a process should largely avoid the formation of deposits and blockages in the hydrolysis apparatus so that long operating times are possible. Further objects of the invention are obvious from the following description.
It has been found that chlorosilanes may be continuously and effectively removed from gas streams if the gas streams are treated in two stages, first, in the gas phase with water vapor, for example steam, and second, with a liquid, aqueous phase.